This study will quantify motion occurring at the socket interface of transhumeral prosthesis users during common tasks. Participants in the study will use a body powered prosthesis with a hosmer hook terminal device. A motion analysis system and a novel optical sensor embedded into the socket wall will record the amount of movement between the residual limb and socket of the prosthesis. Participants will complete range of motion and functional tasks. The range of motion tasks include shoulder flexion, abduction and rotation as well as elbow flexion. The functional tasks include bilateral lift of weighted basket, unilateral lift of weighted handle, box and blocks test, walk and carry a gallon of milk, and folding a towel. It is expected that the more weight at the terminal device the more rotation at the interface, as well as more slip between residual limb and device. It is also expected that the soft tissues of the residual limb can be modeled as a nonlinear spring and the amount of movement at the interface can be predicted based off the force in the system.